1. Field of the Invention
The invention pertains to crowbar circuits for use with high-voltage power supplies particularly for use in printing presses.
2. The Prior Art
It has been recognized that the quality of printing may in certain instances be improved by applying a d.c. voltage, on the order of several kilovolts, between a printing cylinder and an impression roller on a printing press. As the web or sheet material passes through a printing nip between the printing cylinder and the impression roller it is subjected to a strong electrostatic field. An example of one such system is disclosed in U.S. Pat. No. 3,619,720 which issued on Nov. 9, 1971.
A serious problem with applying such high voltages between the impression roller and the printing cylinder, which is normally grounded for safety, is that the impedance between the impression roller and the printing cylinder varies with the material passing through the nip. If the impedance drops excessively arcing results which, because of the vapors due to the links being applied to the web or sheet material, could cause an explosion. Thus, it is important to quickly shut down the high voltage when a decrease in load impedance is detected.
The '720 patent discloses a circuit which senses the load current flowing in a fixed resistor and when that current exceeds a threshold value, the circuit turns on a thyratron which crowbars the output voltage. The output voltage is driven to ground in about 10 microseconds. Experience with the system of the '720 patent has indicated that there exists a need for a faster crowbar system which is activated at lower load currents.